An early method of applying insulation to a conductor or wire was to pass the elongated conductor continuously through a bath of insulating coating material. A more efficient and controlled method of coating wire is extrusion and several methods of extrusion are in use in the plastics industry. The three principle ones are wet extrusion, spinneret extrusion and dry extrusion. Dry extrusion is further divided into direct, semi-positive, and positive types. The machine used for dry extrusion consists essentially of four parts; a feed hopper, a heated cylinder, a revolving screw, and a die. The extrusion compound flows from the feed hopper onto the feed end of the revolving screw, which is within the heated cylinder. It is then forced through the die by the action of the screw in the heated cylinder.
Plastic extrusions are used as primary electrical insulation for wire and cable, as jacketing for these products and to a more limited extent as jacketing for rope and mechanical cable.
Some plastics which find application in this field are PVC and its vinyl acetate copolymer, polyethylene, nylon, polytetrafluoroethylene, polytrifluorochloroethylene, Saran, and silicone rubber.
Representative applications for plastic insulation wire are machine tool and switchboard wiring, portable cord, motor leads, different parts of cable, wiring, and wiring harnesses for the automotive industry, etc.
Standard extrusion machines are used for coating wire, but since it is only practicable to feed the conductor to the machine at an angle, a T-cross head or side delivery head is placed at right angles to the machine with the die fitted into the outlet end of the head. In its simplest form the wire covering cross head includes a wire guide or mandrel and a circumscribing die body. The plastic after leaving the screw, flows over a portion of the outer surface of the wire guide or mandrel which in cooperation, with the inner surface of the die body forms the coating tube. The conductor is passed through the center of the wire guide whereby a hot plastic tube (coating tube) surrounds the conductor and both of them move together through the die. The hot plastic tube engages swaging surfaces in the die causing the hot enveloping plastic tube to abut the conductor. If a tougher assembly is required the coated wire is passed through a braider which applies a braid around the external surface of the assembly and this assembly is again passed through an extruder die, placing another coat of plastic on the braided assembly. Tougher assemblies are utilized where increased cut through resistant characteristics are required.
The complete process for covering a conductor with an insulating coat prior to adding braid includes a pay out drum or reel, an extruder with cross head, capstan and take-up reel.